Bandgap reference circuit

Abstract: According to the present invention, a bandgap reference circuit which utilizes a minimum number of bipolar devices and current mirror scaling devices generates a bandgap reference voltage. The bandgap voltage generated by the bandgap reference circuit is a function of a plurality of sized current mirror devices, the ratio of a first resistor to a second resistor, and the number and relative sizing of bipolar junction transistors used. The bandgap reference circuit generates a bandgap reference voltage which is suitable for use in a variety of integrated circuit devices, such as a zero power static random access memory (SRAM). If used in a zero power SRAM application, the bandgap reference voltage may be utilized to determine when the primary power source of the zero power SRAM has fallen below a predetermined voltage level and a secondary power source must be substituted for the primary power source.

Description: The present invention relates generally to integrated circuits, and more specifically to a MOS bandgap reference circuit. Bandgap reference circuits are used in a variety of integrated circuit devices as a means for sensing changes in the voltage or current level of a device, so that appropriate changes in the operation of the integrated circuit device may be made. In many memory integrated circuit devices, it is desirable to ensure that data stored in the memory is retained and not lost or corrupted upon a loss of power to the device. For example, static random access (SRAM) memory devices referred to as "zero power" devices must be able to sense and respond to changes in the supply voltage. In a zero power SRAM, the data content of the SRAM is protected when the power supply voltage supplied to the SRAM drops below some predetermined voltage level. Typically, the data content of the zero power SRAM is protected by switching from a primary power source to a secondary power source when the power supply voltage to the zero power SRAM falls below the predetermined voltage level. In order to switch from the primary power source to a secondary power source, it is necessary to be able to sense the voltage level of the primary power source and automatically switch to the secondary power source when appropriate. A bandgap reference circuit is one effective means to determine when it is necessary to switch from the primary power source to the secondary power source of a zero power SRAM. However, typical bandgap reference circuits require a large number of bipolar devices which of course consume a large portion of the integrated circuit area of the SRAM. Therefore, because bandgap reference circuits may be effectively used in a zero power SRAM to determine the switching point, it would be desirable to be able to use an improved bandgap reference circuit which has fewer bipolar devices and thus consumes less area and power than bandgap reference circuits currently available. It would be advantageous in the art to utilize a bandgap reference circuit which has fewer bipolar junction transistors than the prior art bandgap reference circuit. It would further be advantageous in the art to utilize a bandgap reference circuit which providing scaling of current through bipolar junction transistors. Therefore, according to the present invention, a bandgap reference circuit which utilizes a minimum number of bipolar devices and current mirror scaling devices generates a bandgap reference voltage. The bandgap voltage generated by the bandgap reference circuit is a function of a plurality of sized current mirror devices, the ratio of a first resistor to a second resistor, and the number and relative sizing of bipolar junction transistors used. The bandgap reference circuit generates a bandgap reference voltage which is suitable for use in a variety of integrated circuit devices, such as a zero power static random access memory (SRAM). If used in a zero power SRAM application, the bandgap reference voltage may be utilized to determine when the primary power source of the zero power SRAM has fallen below a predetermined voltage level and a secondary power source must be substituted for the primary power source. According to the present invention there is provided a bandgap reference circuit, comprising: a first supply voltage;a second supply voltage; a plurality of bipolar junction transistors with each bipolar junction transistor having a collector connected to the first supply voltage, an emitter, and a base, and with a current flowing through each bipolar junction transistor; a first resistor, having a first terminal and a second terminal; means for scaling the current flowing through each bipolar junction transistor, the circuit in use providing a bandgap reference voltage equal to the difference between the first supply voltage and a voltage present at each of the emitters of the bipolar junction transistors; characterised in that said first resistor is located between the first supply voltage and the base of each bipolar junction transistor. Preferred features are set out in the appended claims 2-11. The invention itself, however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: Figure 1 , is a schematic diagram of a bandgap reference circuit, according to the prior art; Figure 2 is a schematic diagram of a bandgap reference circuit, using p-well technology, according to a first preferred embodiment of the present invention; Figure 3 is a schematic diagram of a bandgap reference circuit, using n-well technology, according to a second preferred embodiment of the present invention; Figure 4 is a schematic diagram of a bandgap reference circuit, using p-well technology, according to a third preferred embodiment of the present invention; Figure 4a is a schematic diagram of a cascode connected transistor bandgap reference circuit, using p-well technology, according to a fourth preferred embodiment of the present invention; Figure 5 is a schematic diagram of a bandgap reference circuit which may be used to achieve a lower trip point, according to the present invention; and Figure 6 is a schematic diagram of a circuit which could be used to add hysteresis to the bandgap reference circuit, according to the present invention. Referring to Figure 1 , a schematic diagram of a bandgap reference circuit 1, according to the prior art is shown. Bandgap reference circuit 1 has three bipolar current legs and is comprised of bipolar junction transistors T1, T2, and T3, p-channel transistors P1 and P2, n-channel transistors N1, N2, and N3, and resistors R1 and R2. P-channel transistors P1 and P2, and N-channel transistors N1, N2, and N3 are sized transistors and are selected such that P-channel transistors P1 and P2 have a size ratio of 4:1 to N-channel transistors N1, N2, and N3. This sizing is reflected in Figure 1 by the encircled numbers by the transistors. The current mirror devices N1, N2, and N3 in each of the three bipolar current legs are each set to the same current. Therefore, the current densities of bipolar junction transistors T1 and T3 are approximately ten times that of bipolar junction transistor T2, since the area of bipolar junction transistor T2 is ten times that of T1 and T3, to produce a ln(10) multiplier on the transistor voltage Vt across resistor R1. Additionally, with the same current in each bipolar current leg, resistor R2 must be longer than resistor R1 by a factor of approximately ten times. The present invention employs an improved bandgap reference circuit which generates an actual band gap reference voltage which may be used by zero power circuitry, such as a resistive divider and a comparator, to determine the trip point by matching a fraction of VCC to the bandgap reference voltage. The improved bandgap reference circuit of the present invention offers several advantages over the prior art bandgap reference circuit, including a reduced number of bipolar junction transistors and thus a reduced number of bipolar current legs, and scaled current through the bipolar junction transistors through the use of current mirror devices. Figure 2 shows a schematic diagram of a bandgap reference circuit, using p-well technology, according to a first preferred embodiment of the present invention. Bandgap reference circuit 10 is a simple form of a bandgap reference circuit, according to the present invention, having only two bipolar current legs, and is comprised of bipolar junction transistors T1 and T2, p-channel transistors P1 and P2, n-channel transistors N1, N2, and N3, and resistors R1 and R2. The number of bipolar current legs of Figure 2 over Figure 1 is reduced by placing resistor R2 above the bases of bipolar junction transistors T1 and T2. Bipolar junction transistors T1 and T2 provide Vbe voltage drops at different current densities as indicated by Figure 2; bipolar junction transistor T2 has M emitters whereas bipolar transistor T1 has a single emitter. P-channel transistor P1 and P2 have the same current density and the same source voltages at node a and node b. Therefore, the bipolar current density voltage, v, which appears across resistor R1 is equal to equation 1 which provides: (1) v = kT q ln I3 I1/M = kT q ln M I3 I1 where k is Boltzman's constant, T is the temperature in degrees Kelvin, q is the electronic charge, I3 is the current through p-channel transistor P1, I1 is the current through resistor R1, and M is the number of emitters of bipolar junction transistor T2. N-channel transistors N1, N2, and N3 function as current mirrors that help set the current ratios of I2 and I3 to I1. Therefore, if n-channel transistor N1 has a width w1, n-channel transistor N2 has a width w2, and n-channel transistor N3 has a width w3, then current I2 and current I3 are defined as shown below in equations 2 and 3: (2) I2 = W2 W3 I1 (3) I3 = W1 W3 I1 Thus, the current I1 through resistor R1 is equal to: (4) I1 = v R1 = kT qR1 ln M I3 I1 = kT qR1 ln MW1 W3 The voltage at node c is equal to VCC - I2R2, which when referenced to positive voltage supply VCC is equal to: (5) -I2R2 = - W2 W3 kT q R2 R1 ln MW1 W3 Referenced to VCC, the bandgap reference voltage equation is then: (6) -VOUT = - Vbe - W2 W3 kT q R2 R1 ln MW1 W3 Typical values used for the widths w1, w2, and w3, M, R1, and R2 are such that: (7) MW1 W3 = 20 and W2 W3 R2 R1 = 8 Given these typical values, bandgap reference circuit 10 generates a voltage below VCC of about 1.3 volts. As is evident from the above equations and Figure 2, the present invention offers several important advantages over the prior art bandgap reference circuit 1 of Figure 1. The current through bipolar junction transistors T1 and T2 is scaled using current mirror devices n-channel transistors N1, N2, and N3, such that a desired current density ratio may be achieved with a lower bipolar junction transistor ratio than was possible with the prior art bandgap reference circuit 1 of Figure 1. An additional advantage is that the scaled current I2 through resistor R2 permits resistor R2 to be a lower resistor value. Figure 3 is a schematic diagram of a bandgap reference circuit, using n-well technology, according to a second preferred embodiment of the present invention. Referring to Figure 3, bandgap reference circuit 20 generates a voltage above VCC of about 2.6 volts, roughly double the magnitude generated by Figure 2, because 2 bipolar junction transistors rather than one bipolar junction transistor are used in each bipolar current leg. Bandgap reference circuit 20 is comprised of p-channel transistors P1, P2, P3, P4, and P5 which act as current mirrors, n-channel transistors N1 and N2, source follower bipolar junction transistors T1, T2, T3, and T4, and resistors R1 and R2. Bipolar junction transistors T1, T2, T3, and T4 are sized transistors and are selected such that bipolar junction transistors T1 and T2 have a size ratio of 1:4 with respect to bipolar junction transistors T3 and T4. The sizing is reflected in Figure 3 by the encircled 1's and 4's. Similarly, p-channel transistors are sized transistors and are selected such that p-channel transistors P4 and P5 have a size ratio of 1:5 with respect to p-channel transistors P1 and P2. Thus, for bandgap reference circuitry 20, the current density ratio is 20:1. Therefore, based on the sized transistors, VOUT is defined as follows: (8) VOUT = 2 Vbe + R2 R1 ln (20) kT q A typical value of VOUT is approximately 2.5. Bandgap reference circuit 20 may be made to have better operating characteristics by adding cascode transistors and thereby increase the output impedances of the MOS devices of bandgap reference circuit 20. Such cascode transistors would improve the match between the source follower transistors T1, T2, T3, and T4 and the p-channel transistors which acts as current mirrors. Referring to Figure 4, a schematic diagram of a bandgap reference circuit, using p-well technology, according to a third preferred embodiment of the present invention is shown. Bandgap reference circuitry 30 comprises bipolar junction transistors T1, T2, T3, and T4, p-channel transistors P1 and P2, n-channel transistors N1, N2, N3, N4, and N5, and resistors R1 and R2. Bandgap reference circuitry 30 is analogous to circuitry 20 of Figure 2, except that bandgap reference circuitry 30 is applicable to p-well technology rather than n-well technology. Here, current source n-channel transistors N1, N2, N3, N4, and N5 are still ratioed 1:5 as shown, but are connected to VSS or ground voltage potential. Likewise, bipolar junction transistors T1, T2, T3, and T4 are sized such that the ratio of 1:4 is maintained. The equation for VOUT is analogous to equation (8), except that VOUT is referenced to VCC rather than VSS. If higher drain impedance is desired to provide the current matching required, cascode current mirror circuit, well known in the art, may be added to Figure 4. If higher drain impedance is required to provide the current matching required, a bandgap reference circuit having cascode current mirror circuitry may be used. Referring to Figure 4a , a schematic diagram of a cascode connected transistor bandgap reference circuit 40, using p-well technology, according to a fourth preferred embodiment of the present invention. Cascode connected transistor bandgap reference circuit 40 achieves a similar function to the circuitry of Figure 4, but with greater accuracy. Cascode connected transistor bandgap reference circuit 40 is comprised of bipolar junction transistors T1, T2, T3, and T4, p-channel transistors P1, P2, P3, P4, P5, and P6, n-channel transistors N1, N2, N3, N4, N5, N6, N7, N8, N9, N10, N11, N12, N13, and N14, and resistors R1 and R2. The transistors of Figure 4a are sized as indicated by the encircled numbers. As in Figure 4, Figure 4a utilizes p-well technology. Added cascode current mirror transistors N2, N3, N4, N5, and N6 provide better current matching capabilities while cascode source-follower transistors P3, P4 provide better voltage matching capabilities than the configuration shown in Figure 4. The bias level at point a may be set to keep the cascode current mirror transistors N2, N3, N4, N5, and N6, and current mirror transistors N9, N10, N11, N12, and N13 in saturation. Similarly, the bias level at point b may be set to keep cascode source-follower transistors P3 and P4, as well as source-follower transistors P1 and P2, in saturation. A start-up circuit may be required to establish initial current flow at start-up. The functionality of the bandgap reference circuits shown in Figures 3, 4 and 4a will be adversely impacted as the supply voltage of the circuits is reduced. For example, Vout for the bandgap reference circuit 20 of Figure 3 is approximately 2.5 volts. For operation of the circuit up to 4.5 volts, the Vout to Vcc difference is 2 volts, which permits an ample voltage range for the n-channel source follower above Vout. Since bandgap reference circuit 20 or its p-p-channel equivalent will always be in the bulk, rather than a well, it will have a 2.5 volt back gate bias. Thus, the transistor voltage Vt will be approximately 1.1 volts to 1.5 volts, thereby providing adequate voltage for the current sources to operate. If, however, a lower trip point, such as 4.0 volts, is desired, with operation of the bandgap reference circuit down to 3.5 volts to 3.7 volts, there would not be adequate voltage. Referring to Figure 5 , a schematic diagram of a bandgap reference circuit 50 which may be used to achieve a lower trip point is shown. Bandgap reference circuit 50 is comprised of p-channel transistors P1, P2, and P3, n-channel transistors N1 and N2, bipolar junction transistors T1 and T2, and resistors R1 and R2. The transistors are sized as indicated by the encircled numerals in Figure 5, such that p-channel transistors P1 and P3 have a size ratio of 5:1; n-channel transistors N1 and N2 have a size ratio of 5:1; and bipolar junction transistors T1 and T2 have a size ratio of 1:4. The Vout level would be approximately 1.25 volts and the Vt of the transistors would range from 0.9 volts to 1.2 volts. Thus, at a supply voltage of Vcc = 3.5 volts, the operating voltage for the current source transistors would be equal to 1.05 volts (3.5 volts - 1.25 volts - 1.2 volts). Thus, the bandgap reference circuit 50 of Figure 5 may be used to obtain a considerably lower trip point. Additionally, the bandgap reference circuits described above may be enhanced by adding hysteresis capability. Referring to Figure 6 , a schematic diagram of a circuit 60 which could be used to add hysteresis to a bandgap reference circuit is shown. Circuit 60 is comprised of resistors R1, R2, R3, and R4, transistors T1 and T2, inverters I1 and I2, comparator C, and the Bandgap Reference Circuit shown in the block. Hysteresis can be added by bypassing resistance in the resistor divider with a MOS switch which is controlled by a signal derived from the comparator output C. A positive state at node a indicated that node b is below the trip point. Transistor T2 is turned on and transistor T1 is turned off in order to move node b, thereby producing hysteresis. In Figures 2, 3, 4, 4a and 5, the operation of the bandgap reference circuit is a function of the ratio of resistors R1 and R2, and the ratios of the current mirror and source follower portions of the circuit - not the precise values given resistors R1 and R2. Thus, by carefully choosing the resistor and transistor ratios, a bandgap reference circuit may be realized using few bipolar transistors. The bandgap reference voltage generated by the present invention will typically be used to determine if the bandgap reference voltage is below a predetermined voltage level. If it is below the predetermined voltage level, then a high gain comparator will flip at the predetermined trip point causing the zero power SRAM to be powered by a secondary power source rather than a primary power source. There are several advantages of the present invention over the prior art bandgap voltage circuit. A reduced number of bipolar junction transistors are used according to the present invention, and thus less area is used. Operation of the bandgap reference circuit is dependent on the ratios achieved through careful selection of the values of resistors R1 and R2, as well as the sizes of the transistors, and not on the absolute values of these components. The current mirror devices are scaled such that current going through the bipolar current legs is scaled. Also, multiple trip points can be set by multiplexing multiple taps on the divider. These multiple trip points are chosen to meet customer demands; typical values might be 5%, 10%, 20%, etc. of the value of VCC. Additionally, the present invention allows for VOUT to be brought to an output pin of the zero power SRAM and thus easily measured. While the invention has been particularly shown and described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention.

Claims: A bandgap reference circuit, comprising: a first supply voltage (VCC); a second supply voltage; a plurality of bipolar junction transistors (T1,T2) with each bipolar junction transistor having a collector connected to the first supply voltage, an emitter, and a base, and with a current flowing through each bipolar junction transistor; a first resistor (R2), having a first terminal and a second terminal; means (P 1 ,P 2 ,N 1 ,N 2 ,N 3 ) for scaling the current flowing through each bipolar junction transistor, the circuit in use providing a bandgap reference voltage (Vout) equal to the difference between the first supply voltage and a voltage present at each of the emitters of the bipolar junction transistors; characterised in that said first resistor (R2) is located between the first supply voltage and the base of each bipolar junction transistor. The bandgap reference circuit of claim 1, wherein the means for scaling the current flowing through each bipolar junction transistor is a plurality of current mirror devices (N1, N2, N3) with each current mirror device having a first source/drain, a second source/drain, and a gate, where the first source/drain of each current mirror device is connected to the second supply voltage, the gate of one of said current mirror devices is connected to its second source/drain and the gates of the other current mirror devices are connected to the gate of said one of said current mirror devices. The bandgap reference circuit of claim 2, further comprising: a plurality of bipolar current legs (I1, I2, I3), with a first bipolar current leg having the collector of a first bipolar junction transistor (T1) connected to the first supply voltage, the emitter of the first bipolar junction transistor connected to the second source/drain of a first current mirror device (N1), and the second source/drain of the first current mirror device connected to the second supply voltage; with a second bipolar current leg having the first terminal of the first resistor (R2) connected to the first supply voltage, the second terminal of the first resistor (R2) connected to the second source/drain of a second current mirror device (N2), and the first source/drain of the second current mirror device connected to the second supply voltage; with a third bipolar current leg having the collector of a second bipolar junction transistor (T2) connected to the first supply voltage, the base of the second bipolar junction transistor (T2) connected to the base of the first bipolar junction transistor (T1) and the second terminal of the first resistor, the emitter of the second bipolar junction transistor (T2) connected to a first terminal of a second resistor (R1), a second terminal of the second resistor connected to the second source/drain of a third current mirror device (N3), the gate of the third current mirror device connected to the gate of the second current mirror device (N2), and the first source/drain of the third current mirror device is connected to the second supply voltage; and wherein the first current mirror device, the second current mirror device, and the third current mirror device are sized such that a first current flowing through the first bipolar current leg, a second current flowing through the second bipolar current leg, and a third current flowing through the third bipolar current leg are scaled in relation to each other. The bandgap reference circuit of claim 3 wherein the emitter of the second bipolar junction transistor (T2) is a multiple emitter having M emitters. The bandgap reference circuit of claim 4, wherein the bandgap reference voltage is defined by the following equation: VCC-VOUT = -Vbe - W2 W3 kT q R2 R1 ln MW1 W3 where Vbe is a base emitter voltage of the first bipolar junction transistor, k is Boltzman's constant, T is the temperature in degrees Kelvin, q is the electronic charge, M is the number of emitters of the second bipolar junction transistor, W1 is the width of the first current mirror device, W2 is the width of the second current mirror device, and W3 is the width of the third current mirror device. The bandgap reference circuit of claim 3, wherein the bandgap reference circuit utilizes p-well technology, and the first supply voltage is VCC, the second supply voltage is VSS, and the first current mirror device, the second current mirror device, and the third current mirror device are each an n-channel transistor. The bandgap reference circuit of claim 6, wherein the bandgap reference voltage is approximately 1.3 volts below VCC. The bandgap reference circuit of claim 3, wherein the bandgap reference circuit utilizes n-well technology, and the first supply voltage is VSS, the second supply voltage is VCC, and the first current mirror device, the second current mirror device, and the third current mirror device are each a p-channel transistor. The bandgap reference circuit of claim 3 or claim 8, wherein the bandgap reference voltage is approximately 2.5 volts. The bandgap reference circuit of claim 1 or claim 3, wherein when the bandgap reference voltage falls below a predetermined voltage level, a third supply voltage will be substituted for the first supply voltage. The bandgap reference circuit of claim 10 wherein the bandgap reference circuit is suitable for use in a zero power static random access memory.

Inventors: SLEMMER WILLIAM CARL, SLEMMER, WILLIAM CARL
Applicants: ST MICROELECTRONICS INC, STMICROELECTRONICS, INC.
Application year: 1994
Main classification: G11C5
IPC codes: G05F3, G11C5